The present invention relates to an electrophotographic apparatus of the type using a two-ingredient type developer, i.e., toner and carrier mixture. More particularly, the present invention is concerned with a toner content control device for determining an amount of toner replenishment on the basis of a difference between the toner content of a developer and a reference value, while correcting the reference value in accordance with the condition of a reference pattern formed on a photoconductive element.
An important question with an electrophotographic apparatus of the type described is how the toner content of a developer should be controlled. Japanese Patent Laid-Open Publication No. 1-154179, for example, teaches a toner content control method using a toner content sensor and replenishing toner in such a manner as to maintain a toner content constant. It has been customary with this kind of method to add some circuit for reducing the scattering of the output of the toner content sensor and thereby confining the sensor output in a preselected range.
However, the conventional approach for maintaining the toner content constant has a problem that for a given toner content the density of an image output by the electrophotographic apparatus changes, depending on ambient temperature and humidity and conditions in which the developer is used. This problem is particular to electrophotography which develops a latent image by charging toner by friction.
To solve the above problem, a reference pattern may be formed on a photoconductive element so as to replenish toner such that the density of the reference pattern remains constant, as proposed in the past. Even this kind of scheme has the following problems left unsolved. When the toner content should be noticeably increased in order to guarantee a pattern density in accordance with humidity and temperature and the conditions of use, it is likely that the toner cannot be sufficiently charged and shoots out of a developing unit, smearing the inside of the apparatus. Conversely, when the toner content should be noticeably reduced, the carrier is apt to deposit on the photoconductive element and bring about a trouble. In such a case, it is preferable to confine the toner content in a certain range although the pattern density may slightly deviate from a target density.
In light of the above, there has been proposed to use, in combination with the toner content sensor, a pattern density sensor responsive to the density of a reflection from a reference pattern formed on a photoconductive element. This kind of technology is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2-34877. Specifically, toner is usually so replenished as to maintain the toner content constant in response to the output of the toner content sensor. A reference pattern is formed at preselected intervals or every time a preselected number of copies are produced, while the pattern density sensor senses the density of the reference pattern. A reference value for control and assigned to the toner content sensor is varied in accordance with the sensed pattern density. A reference value associated with the upper and lower limits of toner content is selected beforehand. The reference value is so controlled as not to lie outside of the range between the upper and lower limits even if the pattern density is deviated from a target density. With this scheme, it is possible to implement an electrophotographic apparatus ensuring stable image density.
While the above conventional scheme is capable of stably controlling the image density, it is costly due to the two sensors, i.e., toner content sensor and pattern density sensor. That is, not only the individual sensor is costly, but also an adjusting circuit for the initial setting of the toner content sensor increases the cost.
To reduce the cost, the initial adjustment of the toner content sensor may be omitted. However, if the initial adjustment is simply omitted, then the output of the toner content sensor is scattered with respect to the toner content and makes it impracticable to set the initial reference value and upper and lower limits of the toner content. Moreover, the output curve of the toner content sensor includes a zone in which the sensor output varies only slowly with respect to the varying toner content, i.e., the sensor sensitivity is low. In such a zone, the control itself is impracticable. This is why an adjusting circuit for the initial adjustment has customarily been added to the toner content sensor, at least when pattern density sensor is used in combination with the toner content sensor.
Other conventional approaches for toner density control are disclosed in, e.g., Japanese patent Laid-Open Publication Nos. 3-148679 and 7-333967.